This invention relates to a nondestructive testing apparatus and method, and more specifically to an apparatus and method utilizing parametric generation, acousto-optic reception and signature analysis for analyzing flaws in structures.
It is a continuing effort to produce and design products that are materially sound with a high degree of quality and reliability. With the advent of manufacturing plants and structures designed for operations that involve dangerous or expensive processes the engineering approach of overdesign to include large safety factors became unacceptable and the need arose for a more direct approach for nondestructive evaluation of materials.
Methods of nondestructive evaluation (NDE) include magnetic induction, flux leakage, eddy current, liquid penetrant X-ray radiography and ultrasonic testing. Since the present invention lies in the field of ultrasonic testing of structures, only that field will be addressed.
Forms of nondestructive testing using ultrasonic signals include pulse echo methods and pitch and catch methods. In the pulse echo methods of nondestructive testing, a single transducer, conventionally a piezoelectric crystal material, is utilized to transmit and receive an ultrasonic signals. Piezoelectric crystals are utilized and operated at or near their resonance frequency to enhance their radiation efficiencies and receiving sensitivities. However, most attempts to achieve a uniform output, both in amplitude and phase, over a wide range of frequencies is usually done by damping the resonance of the crystals. This damping results in lower efficiencies in generation and lower sensitivities in reception. Optimally, methods enabling the evaluation and analysis of a large band of frequencies, 0.1 to 10 MHz for example, are desired since the greater number of frequencies available, the more that may be interpreted from the reflected signals.
The pitch and catch methods of ultrasonic testing are fundamentally using the same principals as the pulse echo methods except two transducers are provided; one for transmitting the ultrasonic signal and one for receiving the signal after it has been applied to a test structure. The major disadvantages found in standard ultrasonic devices is in the constraints of the transducers which are not capable of generating and detecting boardband ultrasonic pulses.
It is generally a difficult manufacturing task to make a broadband receiver capable of handling 1 to 10 MHz, the device would be extremely small since the size must be smaller than one wavelength.
The present invention overcomes this disadvantage through the use of a transducer utilizing parametric generation and broadband reception with a method of analysis analogous to novel radar signature analysis.
Studies in experimental scattering in radar indicate that particular waveshapes, such as ramp functions for example, for radiating and interrogating the signal, produce back scattered waveshapes which may be interpreted by a technician to analyze the overall shape of the object detected. In radar scattering studies the information that may be derived from ramp pulse radiating signals includes object cross sectional area, total object volume, and object length along the line of sight. In a nondestructive testing environment the object to be detected in the test structure is the flaw or hole in the test structure analogous to a missle in the atmosphere, for example. Experimental verification of these radar scattering theories are found in a paper entitled "Radar Imaging From Ramp Response Signatures" by Johnathan D. Young, IEEE Transactions on Antennas and Propagation, Vol. AP-24, page 276-282, 1977. The critical obstacle in this method of target identification is in the generation of a proper waveform to interrogate the object to be detected. Since a ramp waveform is desirable, Young in his experimental studies artificially generated a ramp pulse utilizing ten distinct frequency signals.
Analysis of the back scattered signal from the detected object yields information about the detected object beyond mere location. The information interpreted from the waveform height is proportional to the cross sectional area versus the distance along the line of sight of the object. Further, the integral of the waveform is found to be proportional to the total object volume. Also, the object length along the line of sight may be interpreted from the zero crossings of the waveform.
Conventional piezoelectric and other crystal transducers are not capable of generating an ultrasonic signal pulse that is broadband in a useful frequency range. Low frequency envelope functions are known to be generated in a nonlinear absorbtion medium. These absorbtive and nonlinear effects on an ultrasonic signal, produce an effect known as parametric generation or self-demodulation which allows production of a signal at useful frequencies much lower than the frequency of the electrical signal applied to the medium. Typically, an intense ultrasonic frequency signal consisting of an ultrasonic with a defined modulation is generated by a piezoelectric crystal to propagate through an absorbing nonlinear medium. The nonlinear medium will absorb the ultrasonic frequency carrier and the nonlinear interaction will generate a signal of lower frequencies whose shape can be controlled by changing the original modulation function.
The main objective in any nondestructive testing apparatus and method is to provide as much information about the test structure as possible. The test structure may take the form of an oil well casing, a nuclear power reactor, jet and space aircraft, and even the human body. Conventional ultrasonic nondestructive evaluation techniques and apparatus provide information relating only to the location of the defect or flaw. Conventional transducers have an inherent limitation of being narrowband thus reducing the number of frequencies that may be evaluated. Further, useful low frequency signals normally have a spatial spread that is unacceptable to detect flaws that are adjacent to one another. This spatial spread reduces the overall sensitivity of the device and reliability of the test results.